Not surreptitous??

I forgot a word and it's making me crazy. It is neither surreptitious, nor serendipity (though I really thought it was surreptitious). It sounds like both and has a meaning which seems to amalgamete the two. It is used to describe a fortunate yet accidental discovery. I think penicillan, champagne, and sarin nerve gas were "*******ly" discovered. Please solve this riddle.